Exactly what mobo do you have? We can help you check whether it already has an IDE port if you don't know for sure.
The Master / Slave thing can get confusing here. Those terms are used ONLY with respect to one IDE port. The old IDE unit will NOT be a "Slave to the SATA drive". You will have one boot drive (SATA), and a second drive (IDE) used only for data. BUT that IDE unit, being the ONLY device on an IDE port, MUST have its jumper set to the Master position. You see, any IDE port MUST have a Master device on it to work, and it MAY also have a Slave on the same cable. Even if you buy and use an adapter to allow you to connect an IDE drive to a SATA port, basically the adapter creates an IDE port that communicates with the SATA system. In other words, the adapter IS an IDE port and must have a Master device attached to it.
If you're shopping for such an adapter, read the info carefully, because there are two similar units. One allows you to connect a SATA drive to an older IDE port on the mobo, and that is NOT what you want. The other allows connecting an IDE drive to a mobo SATA port. These adapters are NOT two-way - you buy only the right one for your application.